1. Field of the Invention
This invention relates to an active matrix type of liquid-crystal display where thin film transistors (TFTS) arranged in a matrix form are used as switching elements.
2. Description of the Related Art
An active matrix type of TFT liquid-crystal display using TFTs as switching elements has been utilized as a high-quality flat display in a wide variety of applications. In a conventional TFT liquid-crystal display, a liquid-crystal layer is driven by applying an electric field vertical to a substrate between electrodes sandwiching the liquid-crystal layer, typically in a twisted nematic display.
This type of TFT liquid-crystal display, however, has a relatively narrower angle of visibility and a poor contrast, resulting in a whitish screen and a narrow angle of visibility. Thus, there is room for improvement.
JP-As 4-261552 and 6-43461 have suggested techniques for improving visual-angle properties of a liquid-crystal display. In these techniques, a homeotropically oriented liquid-crystal cell is prepared; the cell is sandwiched between two polarizing plates whose polarization axes are perpendicular to each other; a common electrode having an opening is used to generate an oblique electric field in each pixel for dividing each pixel into two or more liquid-crystal domains and thus improving the visual-angle properties. In particular, in JP-A 4-261552, a high contrast can be achieved by controlling a tilting direction of the liquid-crystal during voltage application. In a technique described in JP-A 6-43461, an optical compensator is used as appropriate, to improve visual-angle properties in black. In addition, in JP-A 6-43461, each pixel is divided into two or more domains with an oblique electric field not only in a homeotropically oriented liquid-crystal cell but also in a TN oriented cell, to improve visual-angle properties.
JP-A 5-505247 has suggested an IPS (In-Plane Switching) mode of liquid-crystal display where both of two electrodes are formed on one substrate for allowing a crystal molecule to be rotated while being parallel to the substrate and a voltage is applied between the electrodes to generate an electric field parallel to the substrate. In this type of display, the longitudinal axis of the crystal molecules does not rise in relation to the substrate when applying a voltage. Thus, change in a birefringence of the liquid crystal is small when changing a line of sight, and therefore a wide angle of visibility can be achieved.
There will be described an active matrix type of TFT liquid-crystal display employing an IPS mode where both of two electrodes are formed on one substrate as described above.
FIGS. 5(a)-5(b) show a conventional TFT liquid employing an IPS mode, where FIG. 5(a) is a cross section taken on line A-Axe2x80x2 in FIG. 5(b).
In FIG. 5(a), a gate electrode 502 made of Cr and a common electrode 503 are formed on a glass substrate 501, and a gate insulating film 504 made of silicon nitride is formed, covering these electrodes.
On the gate electrode 502, there is formed a semiconductor film 505 made of amorphous silicon acting as an active layer for a transistor, via the gate insulating film 504. A drain electrode 506 made of molybdenum and a source electrode 507 are superposed on a part of the pattern of the semiconductor film 505, and a protective film 508 made of silicon nitride is formed over all of the elements.
In FIG. 5(b), a region for one pixel is disposed between the source electrode 507 and the extracted common electrode 503.
There is formed an oriented film OR11 which has been rubbed, on the active matrix substrate in which unit pixels constructed as described above are arranged in a matrix form.
On the other hand, a color filter 532 delimited by a shield 533, and a protective film 534 is formed over these elements. Again, an oriented film OR12 which has been rubbed is formed on the protective film 534.
The glass substrate 501 and the opposed substrate 531 are disposed in a manner that their oriented films OR11 and OR12 face to each other, with a space which is filled with a liquid-crystal composition 540. Polarizing plates are formed on the outer faces of the glass substrate 501 and the opposed substrate 531. The shield 533 dividing the color filter 532 is formed such that a part of the shield is superposed over a thin film transistor consisting of the semiconductor film 505.
In a TFT liquid-crystal display constructed as described above, crystal molecules 541a are substantially parallel to the extended electrode direction when an electric field is not applied to the crystal composition 540. Specifically, the liquid-crystal molecules 541a are oriented such that the longitudinal (optical axis) direction of the liquid-crystal molecules 541a forms an angle of at least 45xc2x0 to less than 90xc2x0 to the direction of the electric field generated between the source electrode 507 and the extracted common electrode 503. The mutually facing glass substrate 501 and opposed substrate 531 are parallel to the liquid-crystal molecules. The dielectric anisotropy of the liquid-crystal molecules 541a herein is positive.
On turning on the thin film transistor (TFT) by applying a voltage to the gate electrode 502, a voltage is applied to the source electrode 507 and then an electric field is induced between the source electrode 507 and the opposed common electrode 503. The electric field then changes the direction of the liquid-crystal molecules from 541a to 541b. The liquid-crystal molecules 541b are substantially parallel to the direction of the electric field generated between the source electrode 507 and the opposed common electrode 503.
Since the molecular axis of the liquid-crystal molecules are controlled as described above in an IPS mode, the polarized-light transmitting axis of the polarizing plate 551 may be set at a given angle to change a light transmittance.
As described above, an IPS mode of TFT liquid-crystal display may provide a contrast without a transparent electrode. Furthermore, the longitudinal axis of the liquid-crystal molecules is substantially parallel to the substrate face in the above IPS mode of TFT liquid-crystal display, and therefore, does not rise by voltage application. Thus, in such a display, there may occur a small change in brightness when changing a visual angle, and visual properties may be improved.
Journal of Applied Physics, Vol. 45, No. 12, p.5466 (1974) and JP-A 10-186351 have disclosed, besides the above IPS mode, a mode where a liquid-crystal having a positive dielectric anisotropy is homeotropically oriented to a vertical direction to a substrate and the liquid-crystal molecules are oriented to a direction parallel to the substrate by an electric field parallel to the substrate, where the homeotropically oriented liquid-crystal molecules are divided into two or more regions whose tilting angles are different from each other, due to the direction of the electric field.
However, since a color filter is disposed between a layer on which liquid crystal is placed and an opposed substrate in the above prior art, an electric field generated by applying a voltage between a source electrode and a common electrode affects a color filter, leading to deterioration of display properties for the TFT liquid-crystal display. Pigments in a color filter contain impurities such as sodium ions, causing charging of the color filter when applying an electric field to the filter. The opposed substrate is made of an insulating material. Therefore, when the color is charged, an unwanted electric field may remain in the liquid-crystal under the charged region, which adversely affects the display properties.
To solve the problem, a color filter might be formed on the substrate comprising TFTs (hereinafter, referred to as a xe2x80x9cTFT substratexe2x80x9d) instead on the opposed substrate. There are a variety of interconnecting layers on the TFT substrate. Such a configuration might, therefore, prevent charging of the color filter and reduce effects of accumulated charge in the color filter on the liquid-crystal layer. In addition, the color filter and TFTs formed on the same substrate may make misalignment less frequent in comparison with those formed on separate substrates, allowing a high-definition liquid-crystal display to be favorably fabricated.
However, as there has been increasingly higher level of needs for a high-performance device, it has been needed to eliminate an unwanted electric field to a liquid-crystal layer to a higher level compared to the prior art. For example, for forming a color filter on a TFT substrate, it is necessary to reduce a distance between the color filter and the liquid-crystal layer because of requirements to, e.g., a finer liquid-crystal display. In such a case, a small amount of charge accumulated in the color filter may significantly affect the liquid-crystal layer. In other words, only forming the color filter on the TFT substrate has not been sufficient to meet needs for eliminating an unwanted electric field to a higher level.
Forming a color filter on a TFT substrate is, to some extent, effective for eliminating effects of an unwanted electric field because a pixel electrode and a common electrode which generate an electric field for driving the liquid-crystal are formed on the color filter and thus an unwanted electric field due to charging or charging-up of the color filter is absorbed by the pixel and the common electrodes instead leaking to the liquid-crystal layer. However, attempting to eliminate an unwanted electric field to a higher level, an intense electric field due to scanning and/or signal lines may cause significant charging-up or charging of the color filter, and the electric field may be inadequately absorbed by the electrodes and leak to the liquid-crystal layer. It is, therefore, important to solve the problem. Such charging may deteriorate display properties; for example, uneven coloring may occur in a multicolor liquid-crystal display. Such a problem has become significant by disposing a color-filter on a TFT substrate. It has not been, therefore, recognized in the prior art and there are few investigations for the problem.
In light of the above situation, an objective of this invention is to prevent charging of a color filter due to, for example, an electric field between interconnections and to reduce effects of accumulated charge in the color filter on a liquid-crystal layer in a liquid-crystal display where the color filter is formed on a TFT substrate. Thus, another objective of this invention is to improve display properties of the liquid-crystal display and in particular to prevent uneven coloring in a multicolor liquid-crystal display.
To solve the above problems, this invention provides a liquid-crystal display comprising the first and the second substrates and a liquid-crystal layer between the substrates, wherein a pixel consisting of a pixel electrode and a common electrode in a matrix form and a switching element for controlling operation of the pixel are formed on the liquid-crystal-layer side of the first substrate and a color filter and a shield electrode covering the color filter are formed on the first substrate and the pixel electrode and the common electrode are formed on the shield electrode.
This invention also provides a liquid-crystal display comprising the first and the second substrates and a liquid-crystal layer between the substrates, wherein a pixel consisting of a pixel electrode and a common electrode in a matrix form and a switching element for controlling operation of the pixel are formed on the liquid-crystal-layer side of the first substrate and a color filter is formed on the first substrate and the common electrode is formed covering the color filter.
In a liquid-crystal display of this invention, a color filter is formed on the first substrate and a shield or common electrode covers the color filter. In other words, there are the color filter and the shield or common electrode covering the color filter between the first substrate and the pixel and the common electrodes. Such a structure may allow the color filter to be protected from an electric field generated between interconnections, in particular between the pixel and the common electrodes, and may prevent the color filter from being charged due to these electric fields. There is disposed the shield or common electrode between the color filter and the liquid-crystal layer, which may reduce effects of accumulated charge in the color filter on the liquid-crystal layer. Therefore, according to this invention, display properties of a liquid-crystal display can be improved and uneven coloring can be effectively prevented in a multicolor liquid-crystal display.